Fix NNFE when starting PersistentDataBlock

If the PersistentDataBlockService doesn't have a package configured to
use the block (R.string.config_persistentDataPackageName == "") then we
are currently getting this error:

Started executing PersistentDataBlockService.onStart
 not able to find package
 android.content.pm.PackageManager$NameNotFoundException:
 	at ApplicationPackageManager.getPackageUidAsUser
 	at PersistentDataBlockService.getAllowedUid
 	at PersistentDataBlockService.lambda$onStart

On devices which do not use FRP, this field is blank so we should not
log this message every time.

Test: adb logcat -b all | grep PersistentDataBlockService.lambda
Bug: 168244899
Change-Id: I738f20ad62e5e380181111179efea91cd915faf9